In recent years, there has been an acute need and demand for highly accurate voltage sensors in the electrical power distribution industry. This includes applications in the power grid network, as well those for electrical substations, transformers, switchgear, and relay monitoring.
Historically, measurement of medium voltage at distribution substations has been accomplished using iron-core ferro-magnetic voltage transformers. For instance, technologies such as Rogawski coils (RC), current transformer and power transformer (CT & PT) coils, and inductive voltage dividers have been used for voltage measurement. Such technologies, however, inherently disturb the Electromagnetic Field (EMF) associated with medium voltage transmission measurement. At best, such electromagnetic methods of measurement actively interfere with the voltage to be determined, thereby compromising the measurement of voltage indirectly. These conventional counterparts also have associated risks and danger due to arcing, flash, magnetic saturation, explosions, and catastrophic failure.
Given the challenges inherent to electromagnetic voltage measurement technology, optical sensors have been proposed for medium and high-voltage environments. Such sensors are immune to electromagnetic and radio frequency interference, with no inductive coupling or galvanic connection between the sensor head on high-voltage lines and power transmission substation electronics. The wide bandwidth of optical sensors provides for fast fault and transient detection and power quality monitoring and protection. Optical sensors can be easily installed on, or integrated into, existing substation infrastructure and equipment such as circuit breakers, insulators, or bushings resulting in significant space saving and reduced installation costs with no environmental impact.
Additionally, with the implementation of a Smart Grid and Smart Buildings, there has also been an acute need and demand for new voltage monitors, switch gear, and circuit breakers that can deliver real-time voltage information on low voltage circuits (i.e., <1 kV).
Intelligent switch gear and intelligent circuit breakers will be the key to fully leveraging the Internet of Things (IoT) in managing energy usage both on the grid and within buildings. Conventional circuit breakers function by “tripping,” or shutting off, when voltage or current exceed an upper threshold. The tripping is sufficient for protecting downstream circuitry, machinery, and electronics, but offers no additional control. Intelligent switch gear or circuit breakers, on the other hand, would be able to do significantly more than this simple protection function.
By monitoring voltage, current, volt-ampere reactive (VAR), and total power, intelligent switch gear or circuit breakers can be used to ensure optimal delivery of energy to end-users on the distribution grid. Beyond the grid, intelligent monitoring technology would have broader applications in smart, energy efficient buildings, including office, retail, and industrial buildings, and in transportation systems, including electrical vehicle (EV) charging stations, rail networks, and transportation vehicles.
One key challenge faced by electricity distributors is delivering minimum voltage to all end users. For example, in the United States, distributors are required to deliver 120 volts±5% at 60 Hz to residential users. Thus, residential users must receive between 126 and 114 volts, this being termed “utilization voltage.” Given the way distribution systems are structured, there are drops in this distribution service voltage based on distance between the distribution transformer and the end user. Therefore, in the residential scenario, a first user closer to the distribution transformer may receive 125 volts, whereas a second user further from that same distribution transformer receives only 115 volts. In this scenario, it is crucial that the second user located further from the distribution transformer receives at least 114 volts. While the scenario presented here is for residential users in the United States, it is similar for other end users, or power users, outside of the United States, where distribution service voltages are often higher than 120 V (e.g., 208 V, 240 V).
An important emerging principle in energy management is conservation voltage reduction (CVR). The concept behind CVR is that the distribution transformers provide the minimal voltage possible such that all users on a distribution line receive at least the minimum distribution service voltage (e.g., 114 V for U.S. residential users). Thus, rather than delivering the maximum 126 V to the first residential users on a distribution line to ensure all receive at least 114 V, with CVR distribution service voltage is monitored at all users such that distribution service voltage can be minimized. Doing so allows for optimal grid utilization and one study suggests that for each 1% reduction in distribution service voltage, mean energy consumption is reduced by 0.8%. Thus, optical voltage sensors could also be employed in low voltage applications.
A number of optical voltage sensors that utilize the Pockels effect have been described. For example, U.S. Pat. Nos. 5,731,579; 5,939,711; and 6,492,800, describe Pockels effect-based voltage sensors that include a polarizer at the input and a beam splitter at the output. Devices described by these patents have been deployed by a number of utilities and found to function well at constant temperature. However, when exposed to significant temperature, humidity, and environmental weather swings, these devices were no longer able to accurately monitor voltage.
A major issue in the reliability of optical voltage sensors is the environmental stability, particularly sensitivity due to temperature and humidity of the environment surrounding the optical system or assembly. In previous embodiments of the optical voltage sensors, elaborate polarization diversity schemes have been proposed and utilized that involve various optical components dedicated to polarization manipulation of phase and rotation. However, such polarization components, such as waveplates, retarders, and beam splitters are fragile and can vary greatly over temperature and environmental conditions and change the phase of the optical beam and resultant signal.
Recently, Sima, et al. (2016), “Temperature Characteristics of Pockels Electro-Optic Voltage Sensor With Double Crystal Compensation,” AIP Advances 6, 055109 (2016) described an electro-optic voltage sensor comprised of double, or stacked, LiNbO3 crystals with a complex air spaced polarization diversity scheme. This electro-optic voltage sensor had improved stability from 0° C. to 50° C., however, even this sensor did not meet the temperature stability requirements necessary for monitoring medium and high voltage transmission lines, which require stability from −40° C. to +80° C. Thus, there is a need in the art for a next generation optical voltage sensor, capable of maintaining accurate readings over temperatures ranging from −40° C. to +80° C.
Another key challenge encountered with optical voltage sensor occurs when optical voltage sensors are used in underground or enclosed situations where they may be exposed to flooding or other sources of moisture. In such situations, the standard materials used in optical voltage sensors may be subjected to corrosion, which adversely affects sensor performance and longevity.
The present invention is directed to overcoming these and other deficiencies in the art.